Reasons for rarity of anal melanocytic naevi.

In the last 16 years only seven definite cases of anal melanocytic naevi have been reported in the literature. We describe three new cases, none of which were suspected clinically and were incidentally diagnosed on histological examination of haemorrhoidectomy specimens. The infrequency of these special site flexural melanocytic lesions may be related to changes in the involutionary mechanisms of the anal melanocyte proliferation. However, other factors related to the low detection rates by clinicians and pathologists need also to be considered.